A eutectic is where a mix of chemicals has a locally minimum melting point, i.e. increasing or decreasing the proportion of any of the substances will result in a higher melting point. In most cases, the melting point at the eutectic is lower than the melting point of any of the substances on their own.
Note that a mixture may have more than one eutectic, even if it is of just two components, as the eutectic is just a local minimum (i.e. whichever way you go from the eutectic, the melting point goes up), and not the global minimum (i.e. the lowest possible melting point). However the lowest possible melting point will occur at a eutectic.
As the proportions of components of a mixture are varied, in the solid state they fit together with different levels of ease, and different bond strengths between the components. A eutectic is a point where the bond strengths are at a minimum, and so it is easily melted.

You can read a bit more about eutectics in general on Wikipedia.

Table of Melting Points and Eutectics

The table below gives many eutectic points (in italics) and melting points of substances used by potters, up to 1300°C.
The table is sorted alphabetically based on the formula in additive mode. As an example, soda alumina silicate is given as Na2O·4.550 SiO2·0.185 Al2O3 - we order the molecules first by increasing number of atoms in each molecule, and when this is the same by alphabetical order. Note that older conventions for naming chemicals may place the molecules in a different order.

EutecticNameComposition by weight (%)Melting point °C
KFPotassium fluoride 885
NaFSodium fluoride 1040
CdO·SiO2Cadmium metasilicateCdO 68.0, SiO2 321155
KF·1.2 BaF2Potassium fluoride and barium fluorideKF 78.7, BaF2 21.3750
NaF·2.1 PbF2Sodium fluoride and lead fluorideNaF 7.8, PbF2 92.2540
MnS·13.3 MnSiO3Manganese sulfide and manganese metasilicateMnO 51.5, MnS 4.7, SiO2 43.81130
PbO·0.04 SiO2Lead oxide and silicaPbO 98.8, SiO2 1.2540
PbO·0.20 SiO2Lead oxide and silicaPbO 94.5, SiO2 5.5526
PbO·0.26 SiO2Lead oxide and silicaPbO 92.9, SiO2 7.1508
PbO·0.29 SiO2Lead oxide and silicaPbO 92.2, SiO2 7.8560
3 PbO·SiO2Lead oxide and lead orthosilicatePbO 91.7, SiO2 8.3552
2 PbO·SiO2Lead orthosilicatePbO 88.1, SiO2 11.9746
3 PbO·2 SiO2Lead metasilicate and lead orthosilicatePbO 84.7, SiO2 15.3690
PbO·SiO2Lead metasilicatePbO 78.7, SiO2 21.3770
PbO·2.4 SiO2Lead oxide and silicaPbO 60.6, SiO2 39.4661
NaF·3.28 CaF2·3 AlF3Cryolite and fluoriteNaF 44.8, CaF2 25.4, AlF3 29.8905
BaO·4.0 SiO2·0.43 Al2O3Barium oxide, alumina and silicaBaO 35.0, SiO2 55.0, Al2O3 10.01200
2.5 BaO·2 SiO2·Al2O3Barium aluminium silicate and barium oxideBaO 63.2, SiO2 19.9, Al2O3 16.91220 approx.
CaO·2 SiO2·Al2O3AnorthiteCaO 20.1, SiO2 43.3, Al2O3 36.61550
CaO·2.48 SiO2·0.35 Al2O3Calcium - aluminium - silicateCaO 23.25, SiO2 62.0, Al2O3 14.751170
2 FeO·2 SiO2·Al2O3Ferrous oxide and calcined kaolinFeO 39.2, SiO2 33.0, Al2O3 27.81210 approx.
MgO·2 SiO2·0.392 Al2O3Magnesium oxide, alumina and silicaMgO 20.0, SiO2 60.0, Al2O3 20.01350
2 MgO·2 SiO2·Al2O3Magnesium oxide and calcined kaolinMgO 26.6, SiO2 39.8, Al2O3 33.61330 approx.
2 MnO·2 SiO2·Al2O3Manganese oxide and calcined kaolinMnO 38.9, SiO2 33.1, Al2O3 28.01120 approx.
PbO·1.91 SiO2·0.254 Al2O3Lead - alumina - silicaPbO 61.2, SiO2 31.7, Al2O3 7.1650
PbO·0.78 SiO2·0.238 B2O3Lead oxide - boric oxide - silicaPbO 77.8, SiO2 16.4, B2O3 5.8485
2.5 SrO·2 SiO2·Al2O3Strontium - aluminium - silicate and strontium oxide (SrO)SrO 53.7, SiO2 25.1, Al2O3 21.21290 approx.
ZnO·0.9 SiO2·0.225 Al2O3Zinc oxide, alumina and silicaZnO 51.3, SiO2 34.2, Al2O3 14.51360
3KF·AlF3Potassium cryoliteKF 50.9, AlF3 49.11035
3NaF·AlF3CryoliteNaF 60, AlF3 40998
CaO·Fe2O3Calcium ferriteCaO 26, Fe2O3 741203
0.91 CaO·0.09 Na2O·2.36 SiO2·Al2O31 Albite, 5 AnorthiteCaO 16.9, Na2O 1.9, SiO2 47.3, Al2O3 33.9Fuses 1465 - 1521
0.8 CaO·0.2 Na2O·2.8 SiO2·Al2O31 Albite, 2 AnorthiteCaO 13.7, Na2O 3.8, SiO2 51.3, Al2O3 31.2Fuses 1372 - 1490
0.67 CaO·0.33 Na2O·3.33 SiO2·Al2O31 Albite, 1 AnorthiteCaO 10.4, Na2O5.7, SiO2 55.6, Al2O3 28.3Fuses 1287 - 1450
0.5 CaO·0.5 Na2O·4 SiO2·Al2O32 Albite, 1 AnorthiteCaO 6.9, Na2O 7.7, SiO2 60.1, Al2O3 25.3Fuses 1205 - 1394
0.4 CaO·0.6 Na2O·4.4 SiO2·Al2O33 Albite, 1 AnorthiteCaO 5.2, Na2O 8.7, SiO2 62.2, Al2O3 23.9Fuses 1175 - 1362
0.2 CaO·0.8 Na2O·5.2 SiO2·Al2O38 Albite, 1 AnorthiteCaO 2.4, Na2O 10.5, SiO2 65.6, Al2O3 21.51265
BaO·B2O3Barium metaborateBaO 68.7, B2O3 31.31050
BaO·0.64 B2O3Barium pyroborate and metaborateBaO 77.4, B2O3 22.6750
2 BaO·B2O3Barium pyroborateBaO 81.4, B2O3 18.6Approx 1000
3 BaO·2 B2O3Barium orthoborateBaO 86.8, B2O3 13.2> 1300
CaO·2 B2O3Calcium biborateCaO 28.6, B2O3 71.4Approx. 1025
2CaO·3 B2O3Calcium biborate and metaborateCaO 34.8, B2O3 65.2Approx. 990
CaO·B2O3Calcium metaborateCaO 44.4, B2O3 55.61095
CaO·0.83 B2O3Calcium pyroborate and metaborateCaO 49.1, B2O3 50.9Approx. 1060
2CaO·B2O3Calcium pyroborateCaO 61.5, B2O3 38.5Approx. 1215
CdO·B2O3Cadmium metaborateCdO 64.7, B2O3 35.3875
CuO·B2O3Cupric metaborateCuO 53.2, B2O3 46.8Approx. 980
MgO·B2O3Magnesium oxide and boric oxideMgO 36.6, B2O3 63.4988
2 MgO·B2O3Magnesium oxide and boric oxideMgO 53.6, B2O3 46.41340
3 MgO·B2O3Magnesium oxide and boric oxideMgO 63.5, B2O3 36.51366
4 PbO·B2O3Lead oxide and boric oxidePbO 92.7, B2O3 7.3565
2 PbO·B2O3Lead oxide and boric oxidePbO 86.4, B2O3 13.6497
2.303 PbO·B2O3Lead oxide and boric oxidePbO 88, B2O3 12493
5 PbO·4 B2O3Lead oxide and boric oxidePbO 79.9, B2O3 20.1548
PbO·2 B2O3Lead oxide and boric oxidePbO 61.4, B2O3 38.6768
SrO·2 B2O3Strontium biborateSrO 42.5, B2O3 57.5Approx. 930
SrO·1.58 B2O3Strontium biborate and metaborateSrO 48.2, B2O3 51.8Approx. 890
SrO·B2O3Strontium metaborateSrO 59.7, B2O3 40.31100
SrO·0.71 B2O3Strontium pyroborate and metaborateSrO 67.6, B2O3 32.4980
2 SrO·B2O3Strontium pyroborateSrO 74.7, B2O3 25.3Approx 1115
BaF2Barium fluoride 1280
CaF2Fluorite 1361
PbF2Lead fluoride 855
5 Cu2O·SiO2Cuprous oxide and silicaCu2O 92.23, SiO2 7.771074
4 Cu2O·SiO2Cuprous oxide and silicaCu2O 90.47, SiO2 9.531089
3 Cu2O·SiO2Cuprous oxide and silicaCu2O 87.69, SiO2 12.311062
2 Cu2O·SiO2Cuprous oxide and silicaCu2O 82.61, SiO2 17.391042
Cu2O·SiO2Cuprous oxide and silicaCu2O 70.36, SiO2 29.641048
K2O·1.29 SiO2Potash - silicaK2O 55, SiO2 45780
K2O·7.43 SiO2Potash - silicaK2O 17.5, SiO2 82.5880
Li2O·SiO2Lithium metasilicateLi2O 33.2, SiO2 66.81180
Na2O·0.972 SiO2Soda - silicaNa2O 51.5, SiO2 48.5830
Na2O·SiO2Sodium metasilicateNa2O 50.7, SiO2 49.31018 - 1056
Na2O·4.579 SiO2Soda - silicaNa2O 18.4, SiO2 81.6860
Na2O·1.73 SiO2·1.29 B2O3Soda - boric oxide - silicaNa2O 24.2, SiO2 40.6, B2O3 35.2790
K2O·6 SiO2·Al2O3OrthoclaseK2O 16.9, SiO2 64.8, Al2O3 18.31220
K2O·7.5 SiO2·Al2O32 Orthoclase, 3 silicaK2O 14.5, SiO2 69.8, Al2O3 15.71265
K2O·8 SiO2·Al2O31 Orthoclase, 2 silicaK2O 13.8, SiO2 71.2, Al2O3 15.01285
K2O·9 SiO2·Al2O31 Orthoclase, 3 silicaK2O 12.7, SiO2 73.5, Al2O3 13.81292
K2O·12 SiO2·Al2O31 Orthoclase, 6 silicaK2O 10.2, SiO2 78.7, Al2O3 11.11317
K2O·6.978 SiO2·0.276 Al2O3Potash alumina silicateK2O 17.3, SiO2 77.5, Al2O3 5.2870
K2O·2.85 SiO2·2.11 B2O3Potash - boric oxide - silicaK2O 22.8, SiO2 41.5, B2O3 35.7630
Na2O·4.550 SiO2·0.185 Al2O3Soda alumina silicateNa2O 17.8 SiO2 77.1, Al2O3 5.1800
Na2O·6 SiO2·Al2O3AlbiteNa2O 11.8, SiO2 68.8, Al2O3 19.4Below 1200
Na2O·2 SiO2·Al2O3NephelineNa2O 21.8, SiO2 42.4, Al2O3 35.81223
3 Cu2O·2 B2O3Cuprous sesquiborateCu2O 75.4, B2O3 24.6950 - 1000
KPO3·3.37 (KBO2)Potassium metaborate and KPO3·KBO2K2O 45.7, B2O3 33.9, P2O5 20.4770
KPO3·KBO2Potassium metaborate and metphosphateK2O 47.1, B2O3 17.5, P2O5 35.4885
6.25KPO3·KBO2Potassium metaborate and KPO3·KBO2K2O 41.7, B2O3 4.3, P2O5 54.0681
Li2O·SiO2·4.66(Li2O·B2O3)Lithium metasilicate and metaborate"Li2O 30.9, SiO2 18.7, B2O3 50.4802
Na2O·SiO2·1.52(Na2O·B2O3)Sodium metasilicate and metaborate"Na2O 48.9, SiO2 27.1, B2O3 24.0815
CaF2·1.09 CaSiO3Calcium fluoride and calcium metasilicateCaF2 38.1, CaO 29.8, SiO2 32.11130
K2O·B2O3Potassium metaborateK2O 57.4, B2O3 42.6968
Na2O·B2O3Sodium metaborateNa2O 47.0, B2O3 53965
Na2O·2 B2O3Borax (anhydrous)Na2O 30.7, B2O3 69.3741
K2O·B2O3·1.24 (Na2O·2 B2O3)Potassium and sodium metaboratesK2O 28.7, Na2O 23.5, B2O3 47.8855
Li2O·B2O3·1.43 (Na2O·2 B2O3)Lithium and sodium metaboratesLi2O 10.4, Na2O 30.7, B2O3 58.9640
Na2O·B2O3·Na2O·P2O5Sodium metaborate and sodium metaphosphateNa2O 36.9, B2O3 20.8, P2O5 42.3800
K2O·P2O5Potassium metphosphateK2O 39.9, P2O5 60.1823
Na2O·P2O5Sodium metaphosphateNa2O 30.4, P2O5 69.9610
B2O3Boric oxide 294
MnSiO3Manganese metasilicate (Rhodonite)MnO 54, SiO2 461216
CaSiO3·4 FeSiO3Calcium metasilicate and ferrous silicateFeO 44.6, CaO 8.7, SiO2 32.11130
CaSiO3·7 MnSiO3Calcium and manganese metasilicatesMnO 47.9, CaO 5.4, SiO2 46.71134
1.75 CdSiO3·ZnSiO3Cadmium and zinc metasilicatesZnO 17.2, CdO 47.6, SiO2 35.21052
BaSiO3·2.62 Na2SiO3Barium and sodium metasilicatesNa2O 30.4, BaO 28.7, SiO2 40.9908
CaSiO3·3.8 Na2SiO3Calcium and sodium metasilicatesNa2O 40.6, CaO 9.6, SiO2 49.8932
SrSiO3·5.33 Na2SiO3Strontium and sodium metasilicatesNa2O 40.5, SrO 12.6, SiO2 46.9875
CaO·Al2O3·2SiO2 2.275 (Na2O·Al2O3·2SiO2)Calcium and sodium aluminium silicatesNa2O 15.6, CaO 5.8, SiO2 42.6, Al2O3 361130
0.42 KAlSi3O8·0.58 NaAlSi3O8 approx.Orthoclase and albiteK2O 7.1, Na2O 6.8, SiO2 67.1, Al2O3 19.11160 approx.


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